A certain number of birds are sitting on two trees A and B. If a bird from tree B goes to tree A, the number of birds on the two trees becomes equal. However, if a bird from tree A goes to tree B, the number of birds on tree B becomes twice that on A. How many birds are there on trees A and B respectively?
A3, 5
B7, 6
C5, 7 ✓ Correct
D9, 11
Correct answer: (C) 5, 7
Explanation
Tree A has 5 birds and tree B has 7.
One bird from B to A gives A + 1 = B - 1, so B = A + 2.
One bird from A to B gives B + 1 = 2(A - 1), so B = 2A - 3.
Setting A + 2 = 2A - 3 gives A = 5.
Then B = 5 + 2 = 7.
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